1. Field of the Invention
This invention relates to a liquid mixing method, wherein a small amount of a liquid sample, such as blood, is efficiently mixed with a second liquid, and a uniform mixed liquid is thereby obtained
2. Description of the Prior Art
By way of example, with biochemical analytical apparatuses, a liquid sample, such as blood, is diluted with a predetermined diluent liquid, and a droplet of the resulting mixed liquid is applied to an analytical film having a reagent layer and is thus subjected to a reaction with the reagent layer of the analytical film. In this manner, the content or the activity of a specific chemical constituent, which is contained in the liquid sample, or the content of a physical constituent in the liquid sample is quantitatively analyzed in accordance with the results of the reaction of the chemical or physical constituent (an analyte) contained in the liquid sample and the reagent contained in the reagent layer. In such cases, a small amount (approximately 10 .mu.l) of the liquid sample is diluted with a larger amount (approximately 200 .mu.l) of the diluent liquid, and a predetermined amount (approximately 10 .mu.l) of the resulting mixed liquid is applied to the analytical film.
In general, the stirring of the mixed liquid is carried out by immersing the stirring blade, which is rotated by a motor, in the mixed liquid. However, with such a technique, the stirring blade must be washed each time it has been used for stirring a liquid sample, and facilities for supplying and discharging the washing liquid must be installed. Therefore, the mechanism for the stirring of the mixed liquid cannot be kept simple, and the cost of the mechanism cannot be kept low. Alternatively, the stirring of the mixed liquid may be carried out by giving vibrations to the mixed liquid or to the vessel containing the mixed liquid. As another alternative, the stirring of the mixed liquid may be carried out by blowing a gas, which is inert with respect to the mixed liquid and is clean, to the mixed liquid. However, with these known techniques, the mechanism for the stirring of the mixed liquid cannot be kept simple, and the cost of the mechanism cannot be kept low.
As a method for diluting a liquid sample, a method for diluting a highly viscous liquid has been proposed in, for example, PCT Patent Publication WO 93/07495. The proposed method comprises the steps of introducing a diluent liquid into a vessel, inserting the orifice (i.e., the leading end) of a pipette tip, which contains a liquid sample, into the diluent liquid contained in the vessel, sucking the diluent liquid into the pipette tip, discharging the resulting mixed liquid from the pipette tip into the vessel, repeating the suction and the discharging operation, and thereby mixing the liquid sample and the diluent liquid. With the proposed method, the first stage of the dilution is carried out by utilizing the region inside of the pipette tip.
However, with the conventional liquid mixing method, such as that described above, it is not always possible to efficiently carry out uniform mixing of a liquid sample and a second liquid.
Specifically, in cases where a predetermined amount of a liquid sample is accommodated in a pipette tip and is fed into a mixing vessel, if the amount of the liquid sample to be fed into the vessel is small, a droplet of the liquid sample will be merely formed at the orifice of the pipette tip and cannot be separated and fed from the orifice of the pipette tip. Therefore, it is necessary for a particular operation to be carried out in order to transfer the droplet of the liquid sample into the vessel.
In cases where the second liquid to be mixed with the liquid sample is first fed into the vessel and thereafter the liquid sample is fed into the vessel, the droplet of the liquid sample will be brought into contact with the surface of the second liquid contained in the vessel and will thereby be transferred to the second liquid. However, the height of the surface of the second liquid fluctuates in accordance with a change in the mixing ratio of the second liquid to the liquid sample, a change in the shape of the vessel, and the like. If the height, at which the droplet of the liquid sample is brought into contact with the second liquid, is not adjusted appropriately in accordance with the fluctuation of the height of the surface of the second liquid, the amount of the droplet of the liquid sample, which is transferred to the second liquid, will fluctuate. Therefore, errors in the accuracy, with which the amount of the liquid sample to be fed is measured, become large. The errors are magnified by the mixing ratio of the second liquid to the liquid sample, and the mixing accuracy becomes low as a whole. Also, if the pipette tip for feeding the liquid sample to the second liquid is immersed into the second liquid and is then used to again suck up a new portion of the liquid sample, the problems will occur in that the remaining portion of the liquid sample to be sucked up later is contaminated with the second liquid sticking to the portion of the pipette tip, which was immersed into the second liquid.
In cases where the liquid sample is a blood corpuscle component, or the like, the liquid sample has a high viscosity and a low solubility in the second liquid. Particularly, if the liquid sample is merely fed to the second liquid, little mixing effects will be achieved, and the liquid sample will sediment and stagnate at the bottom of the vessel. Therefore, if the positive mixing operations, such as the suction and discharging operations utilizing the liquid sample pipette tip as in the conventional technique described above, are not carried out, uniform mixing of the liquid sample and the second liquid cannot be achieved. Also, even if the pipette tip is used to suck up the same liquid sample, the pipette tip must be exchanged with a new one after being used to feed the liquid sample to the second liquid. As described above, if the pipette tip is not exchanged with a new one after being used to feed the liquid sample to the second liquid, the problems with regard to the contamination of the liquid sample will occur. Therefore, the liquid mixing operations cannot be kept simple.